This invention relates to controllers for electrically-actuated braking systems, such as those used to apply the brakes on towed vehicles (i.e., trailers) in response to commands from the towing vehicle. More particularly, the invention relates to electronic controllers for braking systems of the general type just noted which operate in response to inertial sensors and/or manually-actuated switches or the like to energize the electric brake-actuation components of such systems in a particular controlled manner.
In the past, electric brake-system controllers have progressed from relatively simple and crude circuits which were little more than manually-variable power switches, operated directly by the driver, to various types of comparatively improved and more sophisticated systems which apply either continuous or pulsing drive excitation to the electromagnetic brake shoe actuators located at the trailer wheels. For example, U.S. Pat. No. 3,738,710 shows a series current regulator which integrates an actuation signal obtained from the towing vehicle brake light circuit and applies continuous braking excitation whose magnitude is basically proportional to the length of time the towing vehicle brakes are actuated, or in any event, proportional to the length of time the brake lights are energized in the towing vehicle. Most other control circuits for electric brakes apply pulsing excitation to the brake-actuating electromagnets, since it is widely thought that such pulsing excitation helps obviate lock-up or skidding of the trailer brakes. Some such controllers utilize a constant pulse-width applied at varying frequencies which increase in accordance with the amount of braking desired, while others utilize a constant-frequency variable-pulse-width form of excitation, for similar reasons. For example, see prior U.S. Pat. Nos. 3,909,075, and 3,953,084, addressed to the second such type of system, together with U.S. Pat. No. 3,967,863, which is directed to the first such type of system, all of which utilize both inertial-sensing and manually-actuatable input devices and apply braking excitation as a function of whichever such device is controlling.
While all of the aforementioned state of the art-type systems no doubt have their individual advantages and favorable features, most also involve certain characteristic limitations or undesirable characteristics. For example, continuous braking excitation is indeed likely to promote trailer brake lock-up, and that is a most undesirable event since it brings about a marked decrease in braking efficiency and loss of operator control. Further, the mere length of time during which the brake light circuit happens to be energized may very well not accurately represent the desirable magnitude of braking force to be applied to the trailer brakes in a given situation. On the other hand, where pulsating brake excitation is utilized, variable-frequency systems usually include some actuation frequencies which unfortunately complement or reinforce resonant frequencies in the vehicle braking systems (whether mechanical, electro-mechanical or electromagnetic in nature), with the result being instability, brake chatter, etc. Indeed, even pulse width-modulated (variable-pulse-width) constant-frequency systems sometimes utilize operational frequencies which have such adverse characteristics, and are likely to have other disadvantages as well.
One common incident of practically all state of the art electronic brake-system controllers is the fact that they utilize, and in fact require, interconnection with the vehicle brake light circuit. This is conventionally felt to be essential in such systems, because it is widely thought that the controller must be kept disabled, i.e., in a condition where it is not capable of providing braking excitation, except for the specific instances when either the manual control switch is actuated or else the towing vehicle brakes are actually being applied, as verified by the presence of the brake light signal. The main reason underlying this conviction is the fact that the stability of prior art inertial sensors and control circuits has not been sufficiently reliable under any and all potential operating conditions to preclude inadvertent and undesired brake actuation under various conditions, for example, in response to such extraneous effects as rough road surfaces, etc.
While using the tow vehicle brake light signal for the purpose just noted did prove to be a reasonably effective measure for coping with the problem of inadvertent brake actuation, this measure nonetheless created a number of problems itself, as well as involving at least some inherent uncertainties. For example, mechanical or electrical failure in the brake light circuit entirely extraneous to actual towing vehicle performance could result in the loss of all trailer braking. Furthermore, with the increasing sophistication of modern-day vehicles, the brake light circuit has grown increasingly complex, since it is now directly intercoupled with such other systems as electronic cruise controls, anti-skid braking systems, etc., and as a result each such system becomes more interdependent and subject to failure or malfunction caused by the others. Furthermore, while cruise controls, anti-skid braking systems, etc., are usually built into the tow vehicle at the factory, this is not true of trailer brake controllers, which are aftermarket devices installed by others. Thus, with the increasing complexity of vehicles and systems related to their brakes and brake-light actuation systems, it becomes increasingly more difficult, as well as more risky and potentially damaging, to physically breach the factory-installed wiring in order to interconnect the brake light circuit with aftermarket devices.
In addition, prior art electronic controllers for electric brake systems have had a number of other disadvantages and limitations, in particular operating inefficiencies attended by the use of excess power and the production of excess heat. Thus, typical prior art systems utilize resistive-type current-sensors for detecting the presence of excess braking current and initiating various forms of interruptors,,for safety purposes, and to prevent controller burn-out. Further, state of the art controllers utilize inefficient drive components such as bi-polar power transistors and the like, thereby using excess power and requiring extensive heat-dissipation means, i.e., heat sinks.